1. Field of the Invention
The present invention relates to an optical disk recording apparatus for recording data on a data-rewritable optical disk.
2. Description of the Related Art
Presently, a disk drive apparatus for recording/reproducing data on a magneto-optical disk having a diameter of 130 mm, which satisfies the ISO standard (ISO)/IEC10089) is commercially available. In the magneto-optical disk drive of this type, a concentrated laser beam is pulse-modulated while applying a constant magnetic field on the magnetic thin film of a magneto-optical disk, and a magnetization reversal domain (pit) is formed selectively on appropriate focalizing spots of the laser beam on the magnetic thin film, thus recording data.
FIG. 1 illustrates the concept of the data recording. As can be seen in this figure, a disk is formed of a transparent substrate 1 and a magnetic thin film 2 having a vertical magnetic anisotropy and formed on the transparent substrate 1. The magnetic thin film 2 initially has a magnetization directed downward as indicated by arrow A. When the power of a concentrated laser beam is increased to a high level pulsewise while applying an external magnetic filed directed upward as indicated by arrow B on the magnetic thin film 2, only the portion whose temperature has been increased by the laser irradiation, changes the direction of its magnetization to the same direction as that of the external magnetic field (indicated by arrow C), and a magnetization reversal domain 4 is formed. As the disk is rotated while the laser beam is applied pulsewise in accordance with the data to be recorded, more magnetization reversal domains 4 are formed successively on the magnetic thin film 2 in accordance with the recording data. Thus, the data is recorded.
As mentioned above, a magneto-optical recording is carried out by increasing the temperature of a magnetic thin film. Therefore, in order to achieve a high-quality recording, the wave height value of a laser of the pulse laser emission, or the pulse height (to be called recording power hereinafter), and the pulse width must be appropriately selected. Note that the pulse width has the same meaning as the duty of the pulse since the pulse emission of the laser is carried out synchronously with the clock-pulse.
FIG. 2 shows the recording power dependency of the error rate in the case where data is recorded, using laser beams having several different pulse widths, on the innermost track of a CAV (Constant Angular Velocity) disk which is in conformity to the ISO standard mentioned above. 1T, 0.75 T and 0.5 T are pulse widths used, and 1T is the pulse width which is equal to the period of the reference clock, so-called channel clock, for recording/reproduction of data. Accordingly, 0.75 T and 0.5 T respectively means 3/4 and 1/2 of the 1T pulse width. The byte-error rate taken in the vertical axis is an error rate of data in unit of 1 byte.
As can be understood from this figure, as the pulse width is narrowed, a higher recording power is required. For a large pulse width such as 1T, the recording power region having a low error rate is narrow, and also the best error rate itself is high as compared to those of other pulse widths. This is because when a recording is carried out with an excessively large pulse width, the diameter of each recording pit is rendered so large that pits adjacent to each other cannot be sufficiently separated. Thus, a write error is likely to occur.
FIG. 3 is a graph showing the results of a similar measurement carried out in the outermost track. Since the radius of this track is larger than the above case, the linear velocity (angular speed) is accordingly higher. Therefore, as compared to FIG. 2, the necessary recording power is shifted to the high power side and the minimum error rate value for the large pulse width of 1T is rendered sufficiently low. This is because the clock frequency (recording frequency) is constant over the entire surface of a disk in the CAV recording carried out in conformity to the ISO standard, the interval between pits is sufficiently large in the periphery portion.
Thus, in the CAV recording, it is not necessary to use a recording beam pulse having a short pulse width of about 0.75 T except for in the innermost track; however, rather, in order to avoid a high recording power being required along with a circumferential speed increased in the peripheral portion, the pulse width of the laser beam is varied in several steps according to the radius position of the track, as shown in FIG. 4. With this recording method, a high-quality recording of data can be performed on the entire surface of a disk.
The above-described technique of varying the pulse width in accordance with the radius position, used in the CAV recording is known and disclosed in, for example, Japanese Patent Publication (KOKAI) 59-24452. In this document, a recording laser pulse modulated (MFM modulation) in accordance with recording data is supplied to a laser drive circuit via a pulse width control circuit. Specifically, the pulse width control circuit comprises a plurality of pulse width conversion circuits having a predetermined period of time for narrowing or expanding a pulse width (for example, a known circuit formed by combining a delay circuit and AND gate or OR gate), and a decoder for converting the data of a radius position into control data used to select one of the plurality of the pulse width conversion circuits.
In this prior art example, there must be provided the same number of pulse width conversion circuits as the number of pulse widths involved. Therefore, in order to achieve a finer control, the circuit structure is rendered accordingly larger. As a result, the size of the overall apparatus is made large, and the production cost becomes high.
Meanwhile, in the CAV recording, the interval between adjacent pits is rendered excessively large as the location of the track becomes closer to the periphery, thus creating an unnecessary recording region. To avoid this, another recording mode is proposed to be used in practice, that is, the pit interval is reduced in outer tracks so as to make the pit interval constant over the entire surface of a disk. Such a recording mode is called ZCAV (Zoned Constant Angular Velocity) recording, and able to achieve a recording of a higher density than that of the Cav recording. In the ZCAV recording, the recording tracks are divided by their radii into a plurality of doughnut-like zones, and the reference clock frequency is switched from one zone to another such that the pit interval is rendered up to its physical limitation (which is substantially the same as the spot diameter determined by the diffraction limit of the concentrated laser beam) in each zone.
FIG. 5 is a table showing values of the reference clock frequency (MHz), 1T (T=1/reference clock frequency), 0.5 T and 0.75T in each zone (zone number, the radius of the innermost track, and that of the outermost track of the zone) of Standard ECMA-184 when rotated at a speed of 1800 rpm, as an example of the ZCAV standard for a disk of 130 mm. As can be understood from the table of this example, the recording density can be increased over the entire surface of a disk and a high-density recording can be achieved by increasing the clock frequency as the location of the zone becomes closer to the periphery.
Also in the ZCAV recording, a pulse width of about 0.75 T should be used in all zones in order to obtain a good byte-error rate in each zone. However, in the ZCAV recording, different reference clock frequencies (1/T) are used in the zones, and therefore in order to generate a pulse of the pulse width of 0.75 T in each zone, the width of the recording pulse must be changed from one zone to another as shown in FIG. 6.
For generating recording pulses having a plurality of pulse widths as above, usually, a delay circuit formed of delay lines and the like is used, as set forth in the above Japanese Patent Publication. FIG. 7 is a block diagram showing an example of the structure of a magneto-optical recording apparatus which can realize the just-mentioned operation.
The magneto-optical recording apparatus includes a controller 11, an SCSI (small computer system interface) interface circuit 12, a recording data pattern generating circuit 13, a reference clock generating circuit 14, a recording pulse generating circuit 15, a laser driver circuit 16 and a laser diode 17.
Data to be recorded, which is output from an external host computer (not shown) is fetched in the SCSI interface circuit 12 under the control of the controller 11, and then input to the recording pattern generating circuit 13. The recording data pattern generating circuit 13 generates a recording data pattern corresponding to the recording data input, and the recording data pattern is output synchronously with the reference clock output by the reference clock generating circuit 14. The reference clock generating circuit 14 generates a reference clock signal whose frequency is set in accordance with a zone being used in recording (zone determined based on the track to which the pick up is set), based on the instruction from the controller 11.
Thus, the recording data pattern which is in synchronous with the reference clock having a predetermined frequency corresponding to a particular zone is input to the recording pulse generating circuit 15. The recording pulse generating circuit 15 includes a flip-flop circuit 15a, a delay circuit 15b formed of delay lines, and a switch 15c formed of analog switches and the like. When a recording data pattern is input, the flip-flop 15a is set at the leading edge of the recording data pattern. The recording data pattern is input also to the delay circuit 15b. The delay circuit 15b has the same number of taps as that of the zones, and each of the taps outputs a signal having a different delay time one from another. The delay time corresponds to 0.75 T of each zone. An output from the delay circuit 15b which corresponds to a recording zone is selected by the switch 15c, and input to the flip-flop 15a as a reset signal. Thus, obtained from the flip-flop 15a, is a recording pulse having a pulse width whose timing corresponding to the delay time given by the delay circuit 15b to a signal selected by the switch 15c using the leading edge of a recording data pattern.
Based on the recording pulse thus generated, the laser diode 17 is turned on pulsewise by the laser drive circuit 16.
According to the above-described structure, the delay time of a signal output from each of the taps of the delay circuit 15b is set at the timing corresponding to 0.75 T of each zone, and therefore a recording can be performed in each zone by using a recording laser pulse whose pulse width is 0.75 T.
However, in order to apply the above-described structure to the ZCAV disk, the same number of taps as that of the zone must be provided on the delay circuit 15b. Usually, there are several tens of zones (as the radius of a disk expands, the number of zones increases), and therefore the structure of the delay circuit 15b is rendered complicated. Further, since one of the outputs of the taps must be selected by the switch 15c, the structure of the switch 15c is also rendered complex. The switching control must be provided for the selection of an output, and the structure becomes more complicated due to the additional structure (not shown) for the control.
As described above, according to the conventional technique, the ZCAV recording by means of a magneto-optical disk entails the following drawback. When the recording pulse width is varied so that data can be recorded with the recording pulse having the optimum pulse width in any of the zones, recording pulses of several different pulse widths must be prepared in advance, and an arbitrary one of the recording pulses is selected for use. However, with such a structure, all of the recording pulse of the different pulse widths must be generated at all times, resulting in a complicated structure.